Immature excitatory synapses in the perinatal brain contain high release probability (Pr) presynaptic terminals coupled to postsynaptic specializations with GluN2B subunit-containing NMDA receptors (hi-Pr, hi-GluN2B synapses). For over two decades, we have known that these immature synapses mature in an activity- dependent manner to low-Pr, low-GluN2B synapses, but the mechanisms that coordinate this transition, why it occurs, and how it contributes to circuit plasticity and stability remain controversial and are fundamental unanswered questions. Addressing these issues will identify basic mechanisms that control synapse development and that may be disrupted in neurodevelopmental and psychiatric disorders. Disruption of the Arg/Abl2 kinase in mice yields a population of hi-Pr, hi-GluN2B synapses that persist into early adulthood. The persistence of these immature synapses drives a >40% net loss of hippocampal synapses between postnatal day (P) 21 and P42, and impairs synaptic plasticity and behavior. Building on these findings, we will identify new regulators of synapse maturation, and determine how they regulate synaptic plasticity and stability. In Aim 1, we will identify the cell surface receptors that activate Arg to coordinate the maturation from hi-Pr, hi- GluN2B synapses to low-Pr, low-GluN2B synapses. We provide preliminary data that integrin ?3?1 adhesion receptor and platelet-derived growth factor receptor ? (PDGFR?) act upstream of Arg to control synapse function and stability. We will use selective gene inactivation in the pre- and postsynaptic neurons along with genetic epistasis and rescue experiments to address how and where these receptors interact with Arg and each other to regulate Pr and postsynaptic GluN2B levels. In Aim 2, we will elucidate how Arg mediates GluN2B downregulation at the synapse. Arg-mediated signaling is critical to downregulate GluN2B during maturation. We identified the SHP2 tyrosine phosphatase and the NMDAR-associated protein BRAG1, both mutated in intellectual disability, as likely functional links between Arg and developmental GluN2B downregulation. We will use biochemical, cell-based, and genetic approaches to test how Arg interacts with SHP2 and BRAG1 to downregulate GluN2B function. In Aim 3, we will characterize how immature and mature synapses differentially contribute to plasticity and stability. We will use patterned glutamate uncaging at single synapses to test whether hi-Pr, hi-GluN2B and low-Pr, low-GluN2B synapses have altered ability to undergo long-term potentiation (LTP) and long-term depression (LTD) in arg?/? mice. We will use in vivo imaging to examine how enlarged dendritic spines at hi- Pr, hi-GluN2B cortical synapses in arg?/? mice differ from normal spines in their plasticity and stability. Our studies will elucidate the mechanisms by which receptors act through Arg and its downstream targets to control Pr and NMDAR composition during synapse maturation. Disruption of these mechanisms may underlie the defects in synapse development, plasticity, and stability in intellectual disability and other brain disorders.